CHAP. X. CLASSIFICATION. 105 



Isopod or an Amphipod before us ; in the newly-hatched 

 young the number of limbs is different, and if we go 

 back to their existence in the egg, the most passing 

 glance to see whether the curvature is upwards or 

 downwards suffices to distinguish even the youngest 

 embryos of the two orders. 



In other instances, the courses which lead from a 

 similar starting-point to a similar goal, separate widely 

 in the middle of the development, as in the Prawns 

 with Nauplius-brood already described. 



Finally, so that even the last possibility may be 

 exhausted, it sometimes happens that the greatest 

 similarity occurs in the middle of the development. 

 The most striking example of this is furnished by the 

 Cirripedia and Ehizocephala, whether we compare the 

 two orders or the members of each with one another ; 

 from a segmentation quite different in its course (see 

 figs. 61-64) proceed different forms of Nauplius, these 

 become converted into exceedingly similar pupae, and 

 from the pupae again proceed sexually mature animals, 

 differing from each other toto coelo. 



" If the formation of the organs occurs in the order 

 corresponding to their importance, this sequence must of 

 itself be a criterion of their comparative value in classifi- 

 cation" THAT IS TO SAY, SUPPOSING THE PHYSIOLOGICAL 

 AND CLASSIFICATIONAL VALUE OF 'AN OKGAN TO CO- 

 INCIDE ! Just as in Christian countries there is a 

 catechismal morality, which every one has upon his 

 lips, but no one considers himself bound to follow, or 

 expects to see followed by anybody else, so also has 



